Excitation, propagation, and damping of electron Bernstein waves in tokamaks

The conventional ordinary O-mode and the extraordinary X-mode in the electr on cyclotron range of frequencies are not suitable for core heating in high -beta spherical tokamak plasmas, like the National Spherical Torus Experime nt [M. Ono, S. Kaye, M. Peng , in Proceedings of the 17th International Ato mic Energy Agency Fusion Energy Conference (International Atomic Energy Age ncy, Vienna, 1999), Vol. 3, p. 1135], as they are weakly damped at high har monics of the electron cyclotron frequency. However, electron Bernstein wav es (EBW) can be effective for heating and driving currents in spherical tok amak plasmas. Power can be coupled to EBWs via mode conversion of either th e X-mode or the O-mode. The two mode conversions are optimized in different regions of the parameter space spanned by the parallel wavelength and wave frequency. The conditions for optimized mode conversion to EBWs are evalua ted analytically and numerically using a cold plasma model and an approxima te kinetic model. From geometric optics ray tracing it is found that the EB Ws damp strongly near the Doppler-broadened resonance at harmonics of the e lectron cyclotron frequency. (C) 2000 American Institute of Physics. [S1070 -664X(00)02710-5].